In advance, using stiff royal icing, figure pipe children. For each figure, on waxed paper, pipe back first, then front. With tip 12, pipe a circle for lower torso; using heavy pressure, pull out legs. Smooth with finger dipped in cornstarch. Pipe a tip 12 circle for upper body, overlapping pants, then pull out arms using heavy pressure. Pipe tip 2 pull-out hair. Let dry completely. Attach lollipop stick to legs of center jumping girl figure. Turn figures over and pipe-in front with tip 12, overpiping stick on center jumping girl. Let dry. Pipe tip 2 ball hands, dot fingers, and shoes and additional pull-out hair. Add tip 2 dot and string facial features.
Ice 2-layer cake smooth. Tint chocolate fondant with black icing color. Roll out 1/4 in. thick and cut an 8 in. diameter circle. Scoop a shallow hole in center of cake and position fondant circle on top. Pipe tip 12 outline top border. Pipe tip 2 c-motion springs from fondant circle to border. Pipe tip 12 outline trampoline legs; cover bottom 1/2 in. of legs with black fondant. Cover cake board with tip 233 pull-out grass. Attach figures around cake with dots of icing. Insert girl figure on lollipop stick in cake center.